Improvement in the detection performance of extremely low-frequency eddy current testing for application in underground steel corrosion detection

Shunki Wakabayashi, Takuya Tomioka, Kenji Sakai, Toshihiko Kiwa, Keiji Tsukada

Research output: Chapter in Book/Report/Conference proceedingConference contribution

1 Citation (Scopus)

Abstract

The corrosion of steel structures, such as light and road sign poles, often occurs near the ground due to rainwater. The subsequent collapse of the corresponding steel structure can cause traffic accidents and potentially danger people. Therefore, a highly accurate and easy nondestructive inspection technique is required to detect corrosion near the ground, especially in hidden locations. Recently, we reported a novel inspection method using extremely low-frequency eddy current testing (ELECT) to detect the reduced steel thickness that was caused by corrosion. Wide magnetic-field exposure was achieved underground using a tilted magnetic sensor probe. Further, the fundamental detection performance and signal analysis of the test samples were also reported. In this study, we attempted to improve the detection performance and applied the measurement to actual steel structures as a field test. The ELECT system to detect corrosion near the ground comprised a magnetic sensor probe, a sensor circuit, a multiple-frequency alternative current (AC) current source for the induction coil, a lock-in detector, and a personal computer. The magnetic probe consisted of two anisotropic magnetic resistance sensors, induction coils to generate eddy currents in the steel structures, and the cancellation coil to reduce the directly coupled magnetic field exposure. The magnetic spectrum curve, which can be used to trace the obtained magnetic field vectors (intensity and phase), was acquired by multiple-frequency exposure. The reduction in steel thickness was further estimated based on the changes in magnetic curve. The optimization of certain parameters, such as the exposure coil shape and lift-off between the sensor probe and the steel, led to improvements in the magnetic curve, which enabled sensitive detection; further, 1-mm thinning was successfully detected at a depth of 60 mm. Furthermore, the results obtained by the ELECT system at the ground surface were in good agreement with the results of the thickness measurements that were directly obtained from the structure after digging into the ground.

Original languageEnglish
Title of host publicationProceedings of the 7th Asia-Pacific Workshop on Structural Health Monitoring, APWSHM 2018
EditorsZhongqing Su, Shenfang Yuan, Hoon Sohn
PublisherNDT.net
Pages512-518
Number of pages7
ISBN (Electronic)9783000603594
Publication statusPublished - Jan 1 2018
Event7th Asia-Pacific Workshop on Structural Health Monitoring, APWSHM 2018 - Hong Kong, China
Duration: Nov 12 2018Nov 15 2018

Publication series

NameProceedings of the 7th Asia-Pacific Workshop on Structural Health Monitoring, APWSHM 2018

Conference

Conference7th Asia-Pacific Workshop on Structural Health Monitoring, APWSHM 2018
CountryChina
CityHong Kong
Period11/12/1811/15/18

Keywords

  • Extremely low-frequency
  • Magnetic spectrum curve
  • Nondestructive testing
  • Underground steel corrosion

ASJC Scopus subject areas

  • Biophysics
  • Mechanical Engineering

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